There are known vitreous ceramic hob plates with circumferentially surrounding frames which are mechanically clipped thereto, the hob plate being embraced along its outer circumferential edge by a silicon-rubber profile strip and the plate, thus provided with the sealing profile, being then screwed fast by mechanical means, such as, e.g., clamping screws, between an overlapping top frame and a base frame or clamping strips on the underside thereof. Such an arrangement presents certain drawbacks; one of these disadvantages is the large amount of material needed for the frame owing to the provision of the second, lower tension frame or the corresponding clamping strips, and the tension or clamping elements, such as springs, screws, clips and their holder means. Another disadvantage arises from the fact that, depending on details of construction of the frame, a great deal of time may be required for its assembly. Another serious technical drawback of this type of assembly resides in that by the very nature of the frame construction involved, it is not possible to preclude faulty assembly work which may result in damage to the hob plate itself. A typical case of this kind is, for example, the fracture of the hob plate caused by localized excessive tensions arising from irregular application of the lower tension frame to the underside of the hob plate.
As far as practical usefulness is concerned, another vital disadvantage resides in that the comparatively thin profiles of the top frame apply clamping tension to the hob plate chiefly along the extreme outer edge regions thereof whereas the overlapping frame part cannot be really firmly pressed against the sealing profile and the hob plate itself. This means that is is impossible to obtain a secure and reliable frame seal. Furthermore, the inner frame edge which rests on the surface of the hob plate also tends to gape slightly in the vertical direction, and because, for reasons of manufacturing tolerances, the leading edge of the silicon rubber profile does not finish perfectly flush with the edge of the overlapping top frame section, dirt which tends to collect in these gaps between frame and hob plate can no longer be completely removed by the user of the appliance. Furthermore, such dirt is liable to swell up and this may give rise to locally higher clamping stresses or tensions which may even lead to fracture of the hob plate.
German Gebrauchsmutern specifications, Nos. 7 401 645 and 7 406 776 describe cooker hob plates which are adhesively fitted in their circumferentially surrounding frames by means of a permanently elastic adhesive material. However, a common feature in these arrangements, and also in all types derived therefrom, is that the circumferentially continuous surround profile does not actually overlap the hob plate surface and for preference the hob plate is adhesively fixed by its underside to an interior fold or flange strip formed by bending, flanging, wrinkling or similar methods or part of the extruded profile, and the top side of the hob plate is either flush with, i.e., in the same plane as the visibly exposed edge of the frame profile or extends from about 1 to 2 mm beneath the exposed frame edge. The gap thus created all around the hob plate between the latter and the frame surround is here filled in with a permanently elastic sealant. While this latter type of arrangement, wherein the hob plates are adhesively fitted in their surrounding frames from the top side thereof, is clearly superior in respect of mechanical tension and stress conditions to the clamped or clip-fitted assemblies and also easier to clean by the user of the appliance, there are still a number of drawbacks regarding manufacture, appearance and choice of design. In the manufacture of this type of hot plate assembly very close tolerances must be observed with regard to the parallelity of the sides of the vitreous ceramic hob plates and to their angularity as well as with regard to the cooperating frame edges. Consequently, expensive and complicated treatment and adhesive fixing technology are required for making the vitreous ceramic hob plates and also expensive production methods for making the surrounding frames. The adhesive fixation method here adopted demands the precise centering of the hob plate in its surrounding frame, since even minor deviations in the parallelity of mutually opposite edges of frame and hob plate will be noticed at a glance and mar the overall appearance of the assembly.
Another drawback of this type of adhesive technique resides in that when using the suitable, conventionally available adhesives which are sufficiently resistant to the thermal loads liable to be applied thereto, there is of necessity a closely restricted choice with regard to the frame materials which may be used in conjunction with such adhesives since there are only a few metals which will form an adequately firm and secure, heat-and food-resisting bond with these adhesives. Furthermore, since the adhesive joint is in the same plane as the hob plate surface, the color of the adhesive material must be matched to the color of the hob plate itself. This imposes considerable limits on design because suitable permanently elastic and heat-stable adhesives which afford an unlimited choice of color are not yet commercially available.